It is important for various reasons to be able to detect an operating state of a vehicle. For example, detecting whether a vehicle is being driven legitimately is important for vehicle security systems. In this case, for example, detecting that a vehicle is moving if the engine is not running could mean that the vehicle is being stolen by being towed or carried away, and then it would be important for the security system to raise an alarm.
However, reliably detecting an operating state of a vehicle engine is not a trivial task, as vehicles nowadays have many different electrical systems, at least some of which may operate even after an engine is switched off. While it is essential to detect that a vehicle is being stolen, for example, it would be highly undesirable to raise false alarms.